Add support for modern ECMAScript class features

With ESLint 8 we should now finally be able to start using modern `class` features, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Public_class_fields and https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields

However, while both ESLint and Acorn now support this, it unfortunately turns out that Escodegen (which we use during building) still lack the necessary support. Looking at https://github.com/estools/escodegen there's not been any updates since last year, and there's also open PRs adding support for these new `class` features.

To avoid blocking usage of these `class` features in the PDF.js code-base, in particular *private* fields/methods, this patch thus proposes that we (hopefully temporarily) switch to an `escodegen` fork that has the necessary support; please see https://www.npmjs.com/package/@javascript-obfuscator/escodegen

While I have no reason to doubt the security of the `escodegen` fork, this patch nonetheless pins the version number. Furthermore, I've also diffed the output of the two `.js`-files in this forked package against the original files without finding anything that looks immediately "dangerous".

gulpfile.js

@@ -1375,7 +1375,7 @@ gulp.task("jsdoc", function (done) {
 gulp.task("types", function (done) {
   console.log("### Generating TypeScript definitions using `tsc`");
   const args = [
-    "target ES2020",
+    "target ESNext",
     "allowJS",
     "declaration",
     `outDir ${TYPES_DIR}`,